Cells synthesize, organize, and remodel the three-dimensional latticework of the extracellular matrix, altering its composition, structure, and ultimately its mechanical properties to fulfill the needs of individual tissues. Understanding (and eventually manipulating) cellular control of extracellular matrix mechanical properties will be essential to the engineering of improved tissue replacements, and will drive the development of new treatment strategies for disorders of the extracellular matrix. Current approaches for investigating cell- mediated control of matrix mechanics are limited by time and resource costs. The central goal of this proposal is to develop, validate, and employ a unique bioassay that enables rapid, cost-effective measurement of the mechanical properties of cell-populated extracellular matrices. The proposed approach, termed magnetic twisting microrheometry, uses micron-scale ferromagnetic beads embedded within cell- matrix constructs (fibroblast-seeded collagen type I gels). The embedded beads are magnetized and exposed to an oscillatory magnetic twisting field. Bead rotations are resisted by the surrounding matrix, to which the beads are firmly coupled. The resulting relationship between applied torque and observed rotation is used to characterize the viscoelastic behavior of the matrix. In the first aim the assay will be developed and optimized, and its output validated against a standard technique. In the second aim magnetic twisting microrheometry will be employed to measure cell-mediated modulation of matrix viscoelastic properties under the control of selected matrix-active and inflammatory mediators, alone and in combination. The proposed assay probes a complex, integrated cellular process on a time and size scale amenable to resource efficient screening of compounds, treatment strategies, and molecular perturbations. Successful completion of the proposed studies could provide a powerful new experimental paradigm by which cell- mediated matrix remodeling is evaluated, setting the stage for detailed investigations into the molecular and microstructural mechanisms by which cells establish and modify extracellular matrix mechanics. The central goal of this proposal is to develop a new technology enabling rapid and efficient dissection of the cellular processes controlling extracellular matrix mechanics. Understanding these processes will improve the engineering of replacement tissues, and the treatment of extracellular matrix diseases. [unreadable] [unreadable]